Rationally Reconstructed Metal-organic Frameworks as Robust Oxygen Evolution Electrocatalysts

Abstract Although the metal-organic framework (MOF) based materials have become one of the most important types of electrocatalysts for the sluggish oxygen evolution reaction (OER), a novel design strategy for the MOF structure is highly needed to overcome the current development bottleneck of the electrochemical performance. Reconstructing MOFs towards a designed framework structure provides breakthrough opportunities to achieve unprecedented OER electrocatalytic performance, but has rarely, if ever, been proposed and investigated yet due to the significant challenges during the synthesis. Here, we report the first successful fabrication of a robust OER electrocatalyst by precision reconstruction of an MOF structure from MOF-74-Fe to MIL-53(Fe)-2OH with different coordination environments at the active sites. Theoretical calculations have revealed that the Fe sites in MIL-53(Fe)-2OH with uncoordinated phenolic hydroxyls are more electroactive than that in MOF-74-Fe. Benefiting from this desired electronic structure, the designed MIL-53(Fe)-2OH catalyst exhibits unprecedentedly high intrinsic OER activity, including a low overpotential of 215 mV at 10 mA cm−2, low Tafel slope of 45.4 mV dec−1 and high turnover frequency (TOF) of 1.44 s−1 at the overpotential of 300 mV, which is 81 times higher than the TOF of the commercial IrO2 catalyst (0.0177 s−1). The radically reduced eg-t2g crystal field splitting in Fe-3d and thus the much suppressed electron hopping barriers through the synergistic effects of the O species from the coordinated carboxyl groups and the uncoordinated phenolic groups guarantee the efficient OER in MIL-53(Fe)-2OH. Consistent with the DFT calculations, the real-time kinetic simulation reveals that the conversion from O* to OOH* is the rate-determining step on the active sites of MIL-53(Fe)-2OH. This work establishes a MOF platform to systematically investigate the structure-property relationship for rationally designing and fabricating robust OER electrocatalysts in the future.